Modern electronic equipment often requires low-ripple, high-current power sources at low to moderate voltages. Conventional switching power supplies can meet these requirements. In addition, switching power supplies are typically more efficient, lighter, and less expensive than their traditional analog counterparts, all of which are advantages in the modern world.
FIG. 1 shows a simplified schematic diagram of a single-phase system 10 utilizing a conventional buck-converter type of switching power supply 11. Power supply 11 incorporates a double-throw switch 12. Switch 12 couples to an inductance 13, and alternately connects a first node of inductance 13 to an input D-C power source 14 and a ground (common) each time switch 12 is toggled. A capacitance 15 and a load 16 are coupled in parallel between a second node of inductance 13 and ground.
Switch 12 is typically realized as a pair of MOSFETs or other active devices operating as double-throw switch 12, and makes a connection in either throw. For the sake of convention, however, this discussion will assume that switch 12 is “on” when it connects inductance 13 to power source 14 and “off” when it connects inductance 13 to ground.
When switch 12 is on, current flows into inductance 13. The energy contained in inductance 13 increases. Current flows from inductance 13 into capacitance 15 and load 16. The energy contained in capacitance 15 also increases. Load 16 receives its energy primarily from inductance 13.
When switch 12 is off, current flows from inductance 13 to ground. The energy contained in inductance 13 decreases. Current flows from capacitance 15 into load 16. Load 16 receives its energy primarily from capacitance 15.
A monitor circuit 17 monitors state variables, such as a voltage across capacitance 15 and a current through inductance 13, to determine when to toggle switch 12. A control circuit 18 controls the switching of switch 12 in response to the state variables monitored by monitor circuit 17.
FIG. 2 shows a simplified schematic diagram of an extension of single-phase system 10 into a multiphase system 10. The following discussion refers to FIG. 2 except as noted therein.
In FIG. 2, system 10 utilizes a multiphase version of switching power supply 11. Often, the high current and/or low ripple requirements of load 16 may be such as to exceed the capabilities of a single switch 12. In such situations, it is common to use N switches 12, where N is an integer greater than one. Each of the N switches 12 couples to its own one of N inductances 13, and connects that one inductance 13 to power source 14 when that switch 12 is on, and to ground when that switch 12 is off. A single power source 14 may be common to all N switches 12. Normally, all inductances 13 couple to a single capacitance 15 and load 16.
Multiphase power supply 11 has N phases 19, where each switch 12 effects one of the N phases 19. Each of the N phases 19 is interleaved with the others. The power dissipated by each switch 12 is a function of the duty cycle of that switch 12. The duty cycle of a given switch 12 is typically maintained at no more than 1/N with N interleaved phases 19. Putting it another way, a symmetrical multiphase system 10 would typically provide approximately N times the current of a single-phase system using the same components for switches 12.
In concept, therefore, there is a significant advantage to multiphase system 10 with a large number of phases 19. However, problems exists with such systems 10 in that, as the number of phases 19 increases, control circuit 18 increases in complexity in order to control and maintain the timing of phases 19. This increase in complexity is reflected in a decrease in reliability and an increase in cost.
One such problem is that each of the N phases 19 should ideally provide approximately the same current. The use of components having typical tolerances may nevertheless result in a wide difference in currents between phases 19, and may result in one switch 12 carrying excessive current. This necessitates that a typical control circuit 18 must manage the individual phase currents, as well as the collective current and the phase timing.
Conventionally, a linear controller is used for control circuit 18. This is a complex circuit requiring inputs from at least N+1 state variables. Moreover, the parameters of a linear controller are tightly matched with the parameters of inductances 13, capacitance 15, and load 16. This often necessitates a change in the controller itself whenever there is even a slight change in inductances 13, capacitance 15, and/or load 16. Consequently, costs associated with control circuit 18 when realized as a linear controller may initially be undesirably high and may be exacerbated by the inability of control circuit 18 to accommodate changes in inductances 13, capacitance 15, and/or load 16.
Control circuit 18 may be realized as a hysteretic controller. Conventional implementations of hysteretic controllers, however, are unsuitable for multiphase systems 10. Even in single-phase systems, hysteretic controllers characteristically exhibit poor performance. This poor performance is due, at least in part, to the inherent lag between the voltage across capacitance 15 and the current through inductances 13. In addition, the switching frequency of switch 12 is dependent upon load 16. That is, the switching frequency will vary as load 16 varies.
Control circuit 18 for system 10 may also be realized as a sliding-mode controller, which may also be viewed as a form of second-order hysteretic controller. Conventional implementations of sliding-mode controllers are also considered unsuitable for multiphase systems 10, but might offer improvements in performance over hysteretic controllers in single-phase systems. With conventional sliding-mode controllers, however, the switching frequency is still dependent upon load 16.
Moreover, simply scaling hysteretic or sliding-mode controllers to manage the phase currents of inductances 13, the collective current, and the phase timing for switches 12 in multiphase system 10 produces no significant improvement in complexity over liner controllers, and does not address the problems of reliability and cost.
Conventional hysteretic and sliding-mode control circuits 18 used in conventional power supplies 11 have switching frequencies that are a function of load 16. That means, as load 16 changes, the switching frequency changes. Since a ripple frequency across capacitance 15, and hence across load 16, is directly related to the switching frequency, changes in load 16 bring about changes in the ripple frequency. The ripple frequency present at load 16 may cause harmonic and/or intermodulation interference with whatever electronic device serves as load 16. Were the ripple frequency to be constant, then the ripple frequency may be chosen to exist in an area of the spectrum to which load 16 is insensitive. Alternatively, relatively simple filtration within load 16 may be used to suppress the effects of the ripple frequency. Allowing ripple frequency to vary makes it difficult to ignore or suppress these effects.
Another problem exists with conventional multiphase switching power supplies utilizing either a hysteretic or sliding-mode control circuit 18 in that, under certain conditions, sudden shifts in load 16 may cause a given switch 12 to enter a lockup condition, i.e., to remain on for an excessive length of time. Under such circumstances, that switch 12 is in danger of exceeding its tolerances and failing. Specifically, the current through that switch 12 for that excessive length of time may cause that switch 12 to exceed its power rating, and may thereby cause a catastrophic failure of that switch 12.
There is a need, therefore, for a switching power supply that has a control circuit that is simple, reliable, and inexpensive, requires a minimal number of state variables, maintains substantially equal current through all inductances, is substantially independent of the tolerances of its components, is immune to variations in the load, is tolerant of switch lockup conditions, and is suitable for either single-phase or multiphase systems.